Objection 1. It would seem that the Blessed Virgin was not sanctified before her birth from the womb. For the Apostle says (1 Corinthians 15:46): "That was not first which is spiritual but that which is natural; afterwards that which is spiritual." But by sanctifying grace man is born spiritually into a son of God according to John 1:13: "(who) are born of God." But birth from the womb is a natural birth. Therefore the Blessed Virgin was not sanctified before her birth from the womb.
Objection 2. Further, Augustine says (Ep. ad Dardan.): "The sanctification, by which we become temples of God, is only of those who are born again." But no one is born again, who was not born previously. Therefore the Blessed Virgin was not sanctified before her birth from the womb.
Objection 3. Further, whoever is sanctified by grace is cleansed from sin, both original and actual. If, therefore, the Blessed Virgin was sanctified before her birth from the womb, it follows that she was then cleansed from original sin. Now nothing but original sin could hinder her from entering the heavenly kingdom. If therefore she had died then, it seems that she would have entered the gates of heaven. But this was not possible before the Passion of Christ, according to the Apostle (Hebrews 10:19): "We have [Vulgate: 'having'] therefore a confidence in the entering into the Holies by His blood." It seems therefore that the Blessed Virgin was not sanctified before her birth from the womb.
Objection 4. Further, original sin is contracted through the origin, just as actual sin is contracted through an act. But as long as one is in the act of sinning, one cannot be cleansed from actual sin. Therefore neither could the Blessed Virgin be cleansed from original sin as long as she was in the act of origin, by existence in her mother's womb.
On the contrary, The Church celebrates the feast of our Lady's Nativity. Now the Church does not celebrate feasts except of those who are holy. Therefore even in her birth the Blessed Virgin was holy. Therefore she was sanctified in the womb.
I answer that, Nothing is handed down in the canonical Scriptures concerning the sanctification of the Blessed Mary as to her being sanctified in the womb; indeed, they do not even mention her birth. But as Augustine, in his tractate on the Assumption of the Virgin, argues with reason, since her body was assumed into heaven, and yet Scripture does not relate this; so it may be reasonably argued that she was sanctified in the womb. For it is reasonable to believe that she, who brought forth "the Only-Begotten of the Father full of grace and truth," received greater privileges of grace than all others: hence we read (Luke 1:28) that the angel addressed her in the words: "Hail full of grace!"
Moreover, it is to be observed that it was granted, by way of privilege, to others, to be sanctified in the womb; for instance, to Jeremias, to whom it was said (Jeremiah 1:5): "Before thou camest forth out of the womb, I sanctified thee"; and again, to John the Baptist, of whom it is written (Luke 1:15): "He shall be filled with the Holy Ghost even from his mother's womb." It is therefore with reason that we believe the Blessed Virgin to have been sanctified before her birth from the womb.
Reply to Objection 1. Even in the Blessed Virgin, first was that which is natural, and afterwards that which is spiritual: for she was first conceived in the flesh, and afterwards sanctified in the spirit.
Reply to Objection 2. Augustine speaks according to the common law, by reason of which no one is regenerated by the sacraments, save those who are previously born. But God did not so limit His power to the law of the sacraments, but that He can bestow His grace, by special privilege, on some before they are born from the womb.
Reply to Objection 3. The Blessed Virgin was sanctified in the womb from original sin, as to the personal stain; but she was not freed from the guilt to which the whole nature is subject, so as to enter into Paradise otherwise than through the Sacrifice of Christ; the same also is to be said of the Holy Fathers who lived before Christ.
Reply to Objection 4. Original sin is transmitted through the origin, inasmuch as through the origin the human nature is transmitted, and original sin, properly speaking, affects the nature. And this takes place when the off-spring conceived is animated. Wherefore nothing hinders the offspring conceived from being sanctified after animation: for after this it remains in the mother's womb not for the purpose of receiving human nature, but for a certain perfecting of that which it has already received.
Source: Summa Theologica (New Advent)